A variety of methods have heretofore been used or proposed for use in applying metallic platings to all or portions of the surfaces of polymeric plastic parts. Such processes conventionally comprise a plurality of sequential pre-treatment steps to render the plastic substrate receptive to the application of an electroless plating whereafter the plated part can be processed through conventional electroplating operations to apply one or a plurality of supplemental metallic platings over all or selected portions of the plastic substrate. Conventionally, the pre-treatment steps employed include a cleaning or series of cleaning steps, if necessary, to remove surface films or contaminating substances followed thereafter by an aqueous acidic etching step employing a hexavalent chromium solution to achieve a desired surface roughness or texture enhancing a mechanical interlock between the substrate and the metallic plating to be applied thereover. The etched substrate is subjected to one or a plurality of rinse treatments to extract and remove any residual hexavalent chromium ions on the surfaces of the substrate which may also include a neutralization step including reducing agents to substantially convert any residual hexavalent chromium ions to the trivalent state. The etched substrate is thereafter subjected to an activation treatment in an aqueous acidic solution containing a tin-palladium complex to form active sites on the surface of the substrate followed by one or more rinsing steps after which the activated surface is subjected to an accelerating treatment in an aqueous solution to extract any residual tin constituents or compounds on the surface of the substrate. The accelerated plastic part is again water rinsed and thereafter is subjected to an electroless plating operation of any of the types known in the art to apply a metallic plate such as copper, nickel or cobalt over all or certain selected areas thereof whereafter the part is rinsed and thereafter is subjected to conventional electroplating operations.
Typical of such plastic plating processes are those described in U.S. Pat. Nos. 3,622,370; 3,961,109; and 3,962,497 to which reference is made for further details of the process. The present invention is also applicable to processes of the foregoing type and is specifically directed to an improved aqueous accelerating solution which provides benefits and advantages heretofore unattainable in accordance with prior art practices.
A continuing problem associated with the electroplating of polymeric substrates has been in the careful control of the activation and accelerating steps to achieve a plastic substrate which is receptive to the subsequent electroless plating solution to provide 100% coverage of a conductive metal layer which is adherent to the substrate and which is devoid of any lack of continuity of coverage or "skipping". The presence of such discontinuities or skips results in plastic parts which upon subsequent electroplating contain non-plated areas or non-uniformity in the metallic plating deposit rendering them unsuitable for the intended end use.
It has been observed that etched and activated plastic substrates employing a tin-palladium complex activator which have not been accelerated or which have been subjected to an accelerating treatment in a weak accelerator will not become plated or will only become partially plated in the subsequent electroless plating step. Such parts are ordinarily referred to as being "under accelerated". On the other hand, when such parts are accelerated in an accelerating solution that is too strong or too aggressive, electroless plating is also adversely effected as evidenced by discontinuity or skips and in some instances no plating deposit at all. In such instances, the parts are referred to as being "over-accelerated". It is important, accordingly, that the accelerating solution employed be carefully controlled so as to provide the requisite degree of acceleration in order to achieve uniform continuous electroless plating deposits on a consistent basis.
It has been observed that accelerating solutions of the types heretofore known are extremely sensitive to the presence of contaminating metal ions carried over from other processing steps or inherently present in the accelerating solution. For example, hexavalent chromium ions in spite of vigorous rinsing and neutralization steps nevertheless are carried over into the subsequent accelerating solution by entrapment in the plastic parts being processed as well as by bleed-through from cracks or openings in the protective plastisol coating conventionally employed over portions of the work racks. Tin compounds similarly are carried over from the prior activation step which adversely affect the accelerating treatment. The presence of ferric and cupric ions in relatively low concentrations such as only 10 ppm and 20 ppm, respectively, have been found to significantly alter the agressiveness of the accelerating solution rendering it unsuitable for further use. Ferric ions constitute a normal contaminant in the water employed for preparing the several aqueous solutions and are further introduced by the dissolution of the stainless steel components of the work rack on which the plastic parts are suspended. Additionally, ferric ions are introduced by oxidation attack of the steel tanks through imperfections in the protective plastic coating which enter the solution and also by conventional rust present in the plating environment. Copper ions similarly are introduced through the water system including copper pipeline, the copper bus bars adjoining the treating receptacles, a dissolution of the rack splines as well as from carryover and bleed-out from the racks as a result of the presence of residual copper on the racks resulting from copper plating operations. Such residual contamination of the racks cannot be completely eliminated in spite of vigorous stripping of the racks at the conclusion of each plating cycle. In many instances, ferric and cupric ion contamination is also introduced as inherent impurities in the chemicals employed to make up the several solutions including the accelerator solution.
In any event, the presence of such ferric cupric, and hexavalent chromium ions in only relatively minimal amounts has adversely effected the accelerating treatment and heretofore has occasioned a discarding and replacement of the aqueous accelerating solution after only a short period of operation.
The present invention overcomes the problems and disadvantages associated with processes for the plating of plastic articles, and particularly the acceleration thereof, by providing a solution which is stable, which is easy to control, which is tolerant to such conventional metallic impurities present, which will further inhibit plating on the protective plastic rack coating, and which is of versatile use over a variety of conventional platable plastic materials.